It is widely practiced to seal electronic devices, for example, transistors, diodes, ICs, capacitors and the like with a synthetic resin to retain their electrical insulating property and avoid variations in physical properties due to the surrounding atmosphere while enjoying its advantages in productivity, cost, etc.
Thermosetting resins such as epoxy resins and silicone resins have conventionally been employed as synthetic sealing resing. They are however pointed out to have drawbacks such that they require a long time for thermosetting and hence a long molding cycle, their storage is not easy because of the need for the prevention of the prematured progress of curing, and sprue runner slug cannot be reused.
Therefore, it has recently been proposed to use, as electronic device sealing resin compositions, resin compositions composed mainly of a polyphenylene sulfide (hereinafter abbreviated as "PPS") which is a thermoplastic resin having excellent heat resistance and flame retardance. Reference may be had, for example, to Japanese Patent Publication No. 9014/1981, Japanese Patent Application Laid-Open No. 17153/1982, Japanese Patent Application Laid-Open No. 21844/1982, Japanese Patent Application Laid-Open No. 40557/1982, Japanese Patent Application Laid-Open No. 20910/1984, Japanese Patent Application Laid-Open No. 20911/1984, Japanese Patent Publication No. 40188/1985, Japanese Patent Application Laid-Open No. 65351/1987, Japanese Patent Application Laid-Open No. 197541/1987 and Japanese Patent Application Laid-Open No. 146963/1988.
A PPS resin however has inferior adhesion to lead frames or bonding wires of electronic devices. When sealing is conducted with a PPS resin composition and the resultant sealed electronic device is placed in a high-humidity atmosphere, moisture tends to penetrate through the interfaces between the sealing resin and its lead frame or bonding wires so that the electrical insulating property may be reduced and/or the lead frame and wires may be corroded and the electrical characteristics of the electronic device may be reduced accordingly.
To cope with such a reduction in moisture resistance, it has been proposed to use a resin composition which is composed mainly of PPS and is blended with silicone oil or silicone rubber, for example, in Japanese Patent Application Laid-Open Nos. 37482/1986, 219145/1988, 219146/1988 and 219147/1988. Japanese Patent Application Nos. 114784/1980 and 38856/1981.
Electronic devices sealed with any of these sealing resin compositions still have insufficient moisture resistance, although some improvements in moisture resistance are observed. They also involve the bleeding problem of silicone oil. Insufficient moisture resistance and silicone oil bleeding have remained as problems to be solved before actual use.